An oxide dielectric thin film having a perovskite structure is expressed by means of the general formula ABO3, and shows excellent ferroelectricity, piezoelectricity, pyroelectricity, and electrooptical property. This attracts attention as an effective material for various kinds of devices such as sensors and actuators, and a range of its application is expected to be rapidly extended in the future. Since a lead zirconate titanate (PZT:general formula is Pb(ZrxTi1-x)O3 (0<x<1)) thin film serving as perovskite oxides has high piezoelectricity, it is used as a piezoelectric element such as a piezoelectric sensor or a piezoelectric actuator. The piezoelectric sensor uses a piezoelectric effect of ferroelectricity. A ferroelectric body has spontaneous polarization inside, and generates positive and negative charges on its surface. In its steady state in the air, the piezoelectric sensor combines with a charge of a molecule in the air to be in a neutral state. When an external pressure is applied to this piezoelectric body, an electric signal can be extracted from the piezoelectric body based on a pressure amount. In addition, the piezoelectric actuator also uses the same principle, and when a voltage is applied to a piezoelectric body, the piezoelectric body expands and contracts based on the voltage, and displacement can be generated in an expansion direction or a direction perpendicular to the expansion direction.
Attempts have been tried to produce the PZT thin film by a vapor-phase growth method represented by a vapor deposition method, sputtering method (sputter method), or CVD method (Chemical Vapor Deposition method), or a liquid-phase growth method represented by a CSD method (Chemical Solution Deposition method), or hydrothermal synthesis method. Among them, the CSD method is easy to control a composition, and easy to produce the thin film with high reproducibility. In addition, as a feature, the CSD method can be low in cost required for a production facility and can be mass-produced.
FIG. 9 is a cross-sectional view of a conventional ferroelectric thin film element. Referring to FIG. 9, thermally oxidized film 12 having a film thickness of 2000 Å is formed on silicon substrate 11, and Ti film 13 having a film thickness of 300 Å is formed on thermally oxidized film 12 by the sputtering method, and Pt film 14 having a film thickness of 2000 Å is also formed on Ti film 13 by the sputtering method, and these films are used as a substrate.
Hereinafter, a method for producing the conventional ferroelectric element will be described.
First, 0.1 mol of lead acetate is added to 1 mol of acetic acid and stirred at 100° C. in a nitrogen atmosphere for about 1 hour. This solution is combined with 36 ml of a solution provided by preparing titanium isopropoxide (Ti(OCH(CH3)2)4) to be 1 mol/L with 2-methoxyethanol and 64 ml of a solution provided by preparing zirconium isopropoxide (Zr(OCH(CH3)2)4) to be 1 mol/L with 2-methoxyethanol. This solution is further stirred at 120° C. in a nitrogen atmosphere for about 3 hours, and cooled down to room temperature and prepared to be 0.5 mol/L with 2-methoxyethanol. Furthermore, 0.2 mol of water is added to this solution and stirred for about 1 hour, and then diethanolamine is added thereto and this is used as a PZT precursor solution. Then, this precursor solution is dropped onto the substrate, and spin-coated under the condition that 350 rpm×3 seconds, and 5000 rpm×20 seconds, and dried gel is produced by a heat treatment at 100° C.×15 minutes. Then, an organic substance is thermally decomposed at 400° C.×60 minutes. These steps are repeated three times, whereby thin film 15 having a film thickness of 2000 Å is obtained. Thin film 15 is subjected to a heat treatment with an infrared rapid thermal annealing (RTA) furnace to be crystallized, whereby PZT thin film 15 is obtained. The heat treatment is performed at atmosphere pressure, in a 100% oxygen atmosphere, at an annealing temperature of 650° C. for an annealing time of 15 seconds. Upper electrode layer 16 is further formed on PZT thin film 15.
FIG. 10 is a view showing polarization-electric field (P-E) hysteresis loops of a conventional piezoelectric element. FIG. 10 shows P-E hysteresis loops showing ferroelectricity before and after repeating 108 polarization inversions of PZT thin film 15 produced by the conventional method. The conventional piezoelectric element is low in linearity of a displacement amount with respect to an applied voltage. This is because a piezoelectric layer is low in crystalline orientation.
This piezoelectric layer is low in squareness M of the P-E hysteresis loop, that is, low in ratio of saturation polarization Ps and remanent polarization Pr expressed such that M=Pr/Ps, and linearity of the displacement amount is low with respect to the applied voltage. When the linearity of the displacement amount is low with respect to the applied voltage, it is difficult to control a device.
Patent document 1 is known as background art document information regarding this application of the present invention.